Ischemic stroke produces rapid profound loss of microvascular integrity. Early following focal ischemia detectable disruption in the permeability barrier of cerebral microvessels occurs, with rapid loss of the endothelial cell 1 integrin-matrix receptors. In amyloid angiopathy, microvessel alterations increase the risk of microhemorrhages. The hypotheses to be tested by this competing renewal Proposal state that i) the interaction of matrix receptors on microvessel endothelial cells (and astrocytes) with matri components of the basal lamina are a major determinant of the blood-brain barrier phenotype, ii) focal cerebral ischemia disrupts receptor-matrix interactions, and iii) interruptions of receptor-matrix interactions result in loss of the blood brain barrier phenotype. We have shown that adhesion of endothelial cells to the intact basal lamina matrix is central to the integrity of the barrier. The barrier phenotype (e.g. tight junctions (TJs)) and vascular matrix are generated by endothelial cells and astrocytes in concert, and are maintained by both cell compartments. The expression of 1 integrins on endothelial cells and -dystroglycan on astrocytes in vivo, and their responses to focal ischemia, are mimicked by primary endothelial cells and astrocytes of murine origin in culture. These responses are matrix-dependent. Based upon successful completed work and preliminary data, we propose that 1 integrins can determine TJ protein expression, and constitute the vertical component of the blood- brain and matrix barriers. The goal of this Project is to demonstrate that the actions of specific 1 integrin family members are required for the integrity of the microvessel barrier, and those specific integrin inhibitors, geneic constructs and knockdowns, focal ischemia, and A1 peptides disrupt the receptor-matrix interactions, producing barrier failure. The Specific Aims are to demonstrate that: 1) the interactions of 1 integrins on confluent endothelium with matrix proteins determine inter-endothelial cell cohesion by the TJ proteins, and these are modulated by astrocytes, 2) mechanisms of 1 integrin signaling within endothelial cells mediate the matrix adhesion receptor-mediated changes in TJ expression (without or with astrocytes), 3) experimental ischemia significantly alters endothelial cell TJ expression by altering the endothelial cell 1 integrin-matrix interactions, and 4) the exposure of endothelial cells (without or with astrocytes) to A-peptides modulates TJ expression and the microvessel permeability barrier phenotype via 1 integrin-dependent mechanisms. These novel studies provide a plausible explanation for the disruption of microvessel integrity immediately following focal ischemia and by amyloid angiopathy. The vertical component mediated by endothelial 1 integrin-matrix adhesion suggests the premise that its disruption is responsible for loss of the blood-brain barrier. Understanding the mechanisms of the 1 integrin-matrix adhesion is likely to lead to new testable approaches to preserve or selectively alter microvessel barrier function in other neurovascular disorders.